You need a TV Tuner card (which will require either a USB port or an open PCI slot) and a relatively decent video card (With TV out (In S-video or component form)).
Expect to spend anywhere from 50-100 dollars on a good tuner card. Most tuner cards require atleast a PII/III or equal AMD processor and 64 MB or RAM. I suggest atleast a 1+ GHz (you should probably have atleast 2, so you can watch TV and do other things at the same time) and probably 256-512 RAM (512 suggested, but than again, 512 is pretty much the performance medium).
Software comes with the retail Tuner Card. Good luck.